In order to reduce energy consumption of artificial illumination sources, the use of high efficiency light sources is increasing, while the use of low efficiency light sources is decreasing. Examples of high efficiency light sources may include gas discharge lamps (e.g., compact fluorescent lamps), phosphor based lamps, high intensity discharge (HID) lamps, light emitting diode (LED) light sources, and other types of high-efficacy light sources. Examples of low efficiency light sources may include incandescent lamps, halogen lamps, and other low efficacy light sources.
Lighting control devices, such as dimmer switches, for example, may allow for controlling the amount of power delivered from a power source to a lighting load, such that the intensity of the lighting load may be dimmed from a high-end (e.g., maximum) intensity to a low end (e.g., minimum) intensity. Both high efficiency and low efficiency light sources may be dimmed, but the dimming characteristics of these two types of light sources may differ.
Due to the increased desire to use more high-efficiency light sources, fluorescent lamps, for example, are now being installed outdoors where the lamps may be subject to low operating temperatures. A ballast may be required to regulate the current conducted through a fluorescent lamp to properly illuminate the lamp. Fluorescent lamps may not operate correctly and may flicker if the lamps are dimmed in cold ambient temperatures. This may be intensified if the lamp has a low mercury concentration. As the lamp is dimmed towards the low-end intensity, the magnitude of a lamp voltage required to drive the lamp may increase. As the temperature of the lamp decreases, the magnitude of the lamp voltage required to drive the lamp may further increase. The increase in lamp voltage required to drive the lamp may cause unnecessary stress on the electrical components of the ballast, as well as instability in the intensity of the lamp near the low end intensity of the lamp, which may consequently produce visible flickering or flashing of the lamp. A load control device for high efficiency light sources that may stably dim a light source to low intensities without flicker in low temperature and/or low mercury conditions may be desired.
FIG. 1 is a perspective view of an example gas discharge lamp fixture 100. The fixture 100 may include a ballast 102, lamp sockets 104, and a housing 106. The ballast 102 and the sockets 104 may be fixed to the housing 106. The lamp sockets 104 may be sized and situated within the housing 106 to hold the lamps 108. The ballast 102 may have wires 110 to connect the ballast 102 to the sockets 104 for driving the lamps 108 and for providing heating current.
FIGS. 2A and 2B show example exterior lamp fixtures 202, 210. These fixtures, typically made of metal or plastic, are particularly suited for outdoor use. In FIG. 2A, the exterior fixture 202 includes a housing 204 and a translucent cover 206. The housing 204 may be mounted to an exterior ceiling or wall. Gas discharge lamps 208 may be attached to the housing via lamp sockets (not shown). A ballast (not shown) may be contained in the housing, as well. Similarly, the fixture 210 shown in FIG. 2B includes a housing 212 and a translucent cover 214. This fixture 210 is shown with a compact fluorescent lamp 216. The compact fluorescent lamp 216 may include an internal ballast contained in the base structure of the lamp. In both examples, the covers 206, 214 may protect the lamps 208, 216 and the ballasts from weather, including water and humidity. However, the lamps and the ballasts may still be subject to the cold ambient temperatures and the corresponding effects described above.
Additional background may be found in commonly assigned U.S. patent application Ser. No. 12/955,988, filed Nov. 30, 2010, entitled METHOD OF CONTROLLING AN ELECTRONIC DIMMING BALLAST DURING LOW TEMPERATURE CONDITIONS, and commonly assigned U.S. patent application Ser. No. 13/629,903 filed Sep. 28, 2012, entitled FILAMENT MISWIRE PROTECTION IN AN ELECTRONIC DIMMING BALLAST, the entire disclosures of each of which are hereby incorporated by reference.